I can think of one other cause - the crap that router vendors peddle to unsuspecting end users.
I fought a Linksys RVL200 VPN router for over a year. It would do VPN semi-decently, but that was about it. Even though it offered the ability to do port forwarding (among other 'features') in the setup menus, the feature didn't consistently work, and trying to run an ATA behind it was almost impossible. I could forward a port and verify that it worked one day, only to find it closed a few days later, with nothing changed in the setup. And this was while running the latest available firmware. It was a complete joke of a router.
I finally flashed a WRT54GS with Tomato (with OpenVPN), and that was the end of my problems. Port forwarding was no longer needed for either of my ATA's, and the VPN feature is like night and day compared to the junk RVL200. Even my data throughput is better.
I suppose this could be classified as 'mis-management on the users setup', but it was caused by the design of the manufacturer. I often wonder how many others are victims of absolute junk foisted on the public by manufacturers that expect us to do their beta testing.
Gary Sanders
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